"Our knowledge of the biting habits of _Simulium_ is not complete, but it is evident, as regards American species at least, that these are sometimes not constant for the same species in different localities.
Certain species will bite man freely when opportunity offers, while others have never been known to attack him. To a.s.sume that the proximity of a _Simulium_-breeding stream necessarily implies that persons in its vicinity must be attacked and bitten is highly fallacious. In Spartanburg County attacks by _Simulium_ seems to be confined to the immediate vicinity of the breeding-places. Our records and observations, exceedingly few in number, refer almost exclusively to such locations.
Statements regarding such attacks, secured with much care and discrimination from a large number of persons, including many pellagrins, indicate conclusively that these insects are seldom a pest of man in this county. A certain number of the persons questioned were familiar with the gnats in other localities, but the majority were seemingly ignorant of the existence of such flies with biting habits.
This is especially striking, in view of the fact that the average distance of streams from the homes of the pellagra cases studied was about 200 yards, many being at a distance of less than 200 yards, and that 78 per cent of these streams were found to be infested by larval _Simulium_. Such ignorance in a large number of persons cannot be overlooked and indicates strongly that our belief in the negligible character of local attacks by _Simulium_ is well founded."
"In localities infested by "sand-flies," mosquitoes, etc., these pests are always well known and the ignorance described above is very significant."
"Such positive reports as we received nearly always referred to bites received in the open, along streams, etc., and observations made of their attack were of those on field laborers in similar situations.
Males engaged in agricultural pursuits are almost exempt from pellagra in Spartanburg County. During the season of 1913, in some two or three instances, observations were made of the biting of _Simulium_ and some additional and entirely creditable reports were received. These observations and reports were under conditions identical with those referred to in the reports of 1912 and confirm the conclusions based on the observations of that year. I would repeat with emphasis that it is inconceivable that a fly of the appearance and habits of the prevalent species of _Simulium_ could be present in such a region, especially about the haunts of man and attack him with sufficient frequency and regularity to satisfactorily account for so active and prevalent a disease as pellagra without being a well-known and recognized pest."
"In connection with the conditions in the Piedmont region of South Carolina, it may be well to cite the results of a study of those in the arid region of western Texas."
"In May, 1913, in company with Capt. J. F. Siler of the Thompson-McFadden Pellagra Commission, I visited the region of which Midland in Midland County is the center. This region is very dry and totally devoid of running water for a long distance in every direction.
The only natural source of water-supply, a few water holes and ponds, were visited and found to be of such a nature that the survival of _Simulium_, far less its propagation in them, is absolutely impossible.
The nearest stream affording possibilities as a source of _Simulium_ is 60 miles away, while the average distance of such possibility is not less than 100 miles."
"Artificial sources of water-supply were also investigated carefully and were found to offer no opportunity for the breeding of _Simulium_."
"At Midland the histories of five cases of pellagra were obtained, which gave clear evidence that this place or its immediate vicinity was the point of origin. Persons of long residence in the country were questioned as to the occurrence of such flies as _Simulium_ and returned negative answers. These included a retired cattle owner, who is a man of education and a keen observer, an expert veterinarian stationed in the country who has the cattle of the country under constant observation, and a practical cattle man, manager of a ranch and of wide experience.
The latter had had experience with "Buffalo gnats" in other localities (in the East) and is well acquainted with them. His close personal supervision of the cattle under his charge, makes it practically certain that he would have discovered these gnats had they been present in the country."
"At the time the study was made, _Simulium_ was breeding and active in the adult state in the vicinity of Dallas, Texas, in the eastern part of the state. We have here a region in which cases of pellagra have originated, yet in which _Simulium_ does not and cannot breed."
Other possible insect vectors of pellagra have been studied in great detail and the available evidence indicates that if _any_ insect plays a role in the spread of the disease, _Stomoxys calcitrans_ most nearly fills the conditions. This conclusion was announced by Jennings and King in 1912, and has been supported by their subsequent work.
Yet, after all the studies of the past decade, the old belief that pellagra is essentially of dietary origin is gaining ground. Goldberger, Waring and Willets (1914) of the United States Public Health Service summarize their conclusions in the statement, (1) that it is dependent on some yet undetermined fault in a diet in which the animal or leguminous protein component is disproportionately large and (2) that no pellagra develops in those who _consume_ a mixed, well-balanced, and varied diet, such, for example, as that furnished by the Government to the enlisted men of the Army, Navy, and Marine Corps.
Leprosy
LEPROSY is a specific, infectious disease due to _Bacillus leprae_, and characterized by the formation of tubercular nodules, ulcerations, and disturbances of sensation. In spite of the long time that the disease has been known and the dread with which it is regarded, little is known concerning the method of transfer of the causative organism or the means by which it gains access to the human body.
It is known that the bacilli are to be found in the tubercles, the scurf of the skin, nasal secretions, the sputum and, in fact in practically all the discharges of the leper. Under such conditions it is quite conceivable that they may be transferred in some instances from diseased to healthy individuals through the agency of insects and other arthropods. Many attempts have been made to demonstrate this method of spread of the disease, but with little success.
Of the suggested insect carriers none seem to meet the conditions better than mosquitoes, and there are many suggestions in literature that these insects play an important role in the transmission of leprosy. The literature has been reviewed and important experimental evidence presented by Currie (1910). He found that mosquitoes feeding, under natural conditions, upon cases of nodular leprosy so rarely, if ever, imbibe the lepra bacillus that they cannot be regarded as one of the ordinary means of transference of this bacillus from lepers to the skin of healthy persons. He believes that the reason that mosquitoes that have fed on lepers do not contain the lepra bacillus is that when these insects feed they insert their proboscis directly into a blood vessel and thus obtain bacilli-free blood, unmixed with lymph.
The same worker undertook to determine whether flies are able to transmit leprosy. He experimented with five species found in Honolulu,--_Musca domestica_, _Sarcophaga pallinervis_, _Sarcophaga barbata_, _Volucella obesa_ and an undetermined species of _Lucilia_.
The experiments with _Musca domestica_ were the most detailed. From these experiments he concluded, first, that all of the above-named flies, when given an opportunity to feed upon leprous fluids, will contain the bacilli in their intestinal tracts and feces for several days after such feeding. Second, that considering the habits of these flies, and especially those of _Musca domestica_, it is certain that, given an exposed leprous ulcer, these insects will frequently convey immense numbers of lepra bacilli, directly or indirectly, to the skins, nasal mucosa, and digestive tracts of healthy persons. Additional evidence along this line has recently been brought forward by Honeij and Parker (1914), who incriminate both _Musca domestica_ and _Stomoxys calcitrans_. Whether or not such insect-borne bacilli are capable of infecting persons whose skin and mucosa are thus contaminated, Currie was unwilling to maintain, but he concludes that until we have more accurate knowledge on this point, we are justified in regarding these insects with grave suspicion of being one of the means of disseminating leprous infection.
Various students of the subject have suggested that bed-bugs may be the carriers of leprosy and have determined the presence of acid-fast bacilli in the intestines of bed-bugs which had fed on leprous patients.
Opposed to this, the careful experiments of Thompson (1913) and of Skelton and Parkham (1913) have been wholly negative.
Borrel has recently suggested that _Demodex_, may play a role in spreading the infection in families. Many other insects and acariens have been suggested as possible vectors, but the experimental data are few and in no wise conclusive. The most that can be said is that it is quite possible that under favorable conditions the infection might be spread by any of the several blood-sucking forms or by house-flies.
Verruga peruviana
VERRUGA PERUVIANA is defined by Castellani and Chalmers as "a chronic, endemic, specific, general disorder of unknown origin, not contagious, but apparently inoculable, and characterized by an irregular fever a.s.sociated with rheumatoid pains, anemia, followed by granulomatous swellings in the skin, mucous membranes, and organs of the body." It has been generally believed by medical men interested that the comparatively benign eruptive verruga is identical with the so-called Oroya, or Carrion"s fever, a malignant type. This view is not supported by the work of Strong, Tyzzer and Brues, (1913).
The disease is confined to South America and to definitely limited areas of those countries in which it does occur. It is especially prevalent in some parts of Peru.
The causative organism and the method of transfer of verruga are unknown. Castellani and Chalmers pointed out in 1910 that the study of the distribution of the disease in Peru would impress one with the similarity to the distribution of the Rocky Mountain fever and would lead to the conclusion that the aetiological cause must in some way be a.s.sociated with some blood-sucking animal, perhaps an arachnid, and that this is supported by the fact that the persons most p.r.o.ne to the infection are those who work in the fields.
More recently, Townsend (1913), in a series of papers, has maintained that verruga and Carrion"s disease are identical, and that they are transmitted to man by the bites of the Psychodid fly, _Phlebotomus verrucarum_. He succeeded in producing the eruptive type of the disease in experimental animals by injecting a physiological salt trituration of wild Phlebotomus flies. A cebus monkey was exposed from October so to November 6, by chaining him to a tree in the verruga zone, next to a stone wall from which the flies emerged in large numbers every night.
Miliar eruption began to appear on the orbits November 13 and by November 21, there were a number of typical eruptions, with exudation on various parts of the body exactly like miliar eruptive sores commonly seen on legs of human cases.
An a.s.sistant in the verruga work, George E. Nicholson, contracted the eruptive type of the disease, apparently as a result of being bitten by the Phlebotomus flies. He had slept in a verruga zone, under a tight net. During the night he evidently put his hands in contact with the net, for in the morning there were fifty-five unmistakable Phlebotomus bites on the backs of his hands and wrists.
Townsend believes that in nature, lizards const.i.tute the reservoir of the disease and that it is from them that the Phlebotomus flies receive the infection.
Cancer
There are not wanting suggestions that this dread disease is carried, or even caused, by arthropods. Borrel (1909) stated that he had found mites of the genus _Demodex_ in carcinoma of the face and of the mammae. He believed that they acted as carriers of the virus.
Saul (1910) and Dahl (1910) go much further, since they attribute the production of the malignant growth to the presence of mites which Saul had found in cancers. These Dahl described as belonging to a new species, which he designated _Tarsonemus hominis_. These findings have since been confirmed by several workers. Nevertheless, the presence of the mite is so rare that it cannot be regarded as an important factor in the causation of the disease. The theory that cancer is caused by an external parasite is given little credence by investigators in this field.
IN CONCLUSION, it should be noted that the medical and entomological literature of the past few years abounds in suggestions, and in unsupported direct statements that various other diseases are insect-borne. Knab (1912) has well said "Since the discovery that certain blood-sucking insects are the secondary hosts of pathogenic parasites, nearly every insect that sucks blood, whether habitually or occasionally, has been suspected or considered a possible transmitter of disease. No thought seems to have been given to the conditions and the characteristics of the individual species of blood-sucking insects, which make disease transmission possible."
He points out that "in order to be a potential transmitter of human blood-parasites, an insect must be closely a.s.sociated with man and normally have opportunity to suck his blood repeatedly. It is not sufficient that occasional specimens bite man, as, for example, is the case with forest mosquitoes. Although a person may be bitten by a large number of such mosquitoes, the chances that any of these mosquitoes survive to develop the parasites in question, (a.s.suming such development to be possible), and then find opportunity to bite and infect another person, are altogether too remote. Applying this criterion, not only the majority of mosquitoes but many other blood-sucking insects, such as Tabanidae and Simuliidae, may be confidently eliminated. Moreover, these insects are mostly in evidence only during a brief season, so that we have an additional difficulty of a very long interval during which there could be no propagation of the disease in question." He makes an exception of tick-borne diseases, where the parasites are directly transmitted from the tick host to its offspring and where, for this reason, the insect remains a potential transmitter for a very long period. He also cites the trypanosome diseases as possible exceptions, since the causative organisms apparently thrive in a number of different vertebrate hosts and may be transmitted from cattle, or wild animals, to man.
Knab"s article should serve a valuable end in checking irresponsible theorizing on the subject of insect transmission of disease.
Nevertheless, the principles which he laid down cannot be applied to the cases of accidental carriage of bacterial diseases, or to those of direct inoculation of pyogenic organisms, or of blood parasites such as the bacillus of anthrax, or of bubonic plague. Acc.u.mulated evidence has justified the conclusion that certain trypanosomes pathogenic to man are harbored by wild mammals, and so form an exception. Townsend believes that lizards const.i.tute the natural reservoir of verruga; and it seems probable that field mice harbor the organism of tsutsugamushi disease.
Such instances are likely to acc.u.mulate as our knowledge of the relation of arthropods to disease broadens.
CHAPTER XII
HOMINOXIOUS ARTHROPODS
The following synoptic tables are presented in the hope that they may be of service in giving the reader a perspective of the relationships of the Arthropoda in general and enabling him to identify the more important species which have been found noxious to man. Though applicable chiefly to the arthropods found in the United States, exotic genera and species which are concerned in the transmission of disease are also included. For this reason the keys to the genera of the Muscids of the world are given. As will be seen, the tables embrace a number of groups of species which are not injurious. This was found necessary in order that the student might not be lead to an erroneous determination which would result were he to attempt to identify a species which heretofore had not been considered noxious, by means of a key containing only the noxious forms. The names printed in BOLD FACED TYPE indicate the hominoxious arthropods which have been most commonly mentioned in literature.
CRUSTACEA
Arthropods having two pairs of antennae which are sometimes modified for grasping, and usually with more than five pairs of legs. With but few exceptions they are aquatic creatures. Representatives are: Crabs, lobsters, shrimps, crayfish, water-fleas, and woodlice. To this cla.s.s belongs the CYCLOPS (fig. 122) a genus of minute aquatic crustaceans of which at least one species harbors _Dracunculus medinensis_, the Guinea worm (fig. 121).
MYRIAPODA
Elongate, usually vermiform, wingless, terrestrial creatures having one pair of antennae, legs attached to each of the many intermediate body segments. This group is divided into two sections, now usually given cla.s.s rank: the DIPLOPODA or millipedes (fig. 13), commonly known as thousand legs, characterized by having two pairs of legs attached to each intermediate body segment, and the CHILOPODA or centipedes (fig.
14) having only one pair of legs to each body segment.
ARACHNIDA